These experiments are designed to find out how muscles work as a highly efficient and integrated converter of chemical potential energy ("high energy phosphate compounds", approximately P) into useful work. Two intertwined and complementary aspects are measured: (1) The utilization of approximately P during contractile activity and (2) The control of metabolic generation and storage of approximately P. Different energetic and metabolic strategies of skeletal and cardiac muscles (rodent EDL and SOL, amphibian, cat biceps brachii and papillary) will be exploited to sort out factors determining and regulating both aspects of energy transformation. Measurements of the initial approximately P breakdown by rapid-freezing techniques, of recovery oxygen consumption and lactate production, and of ATPase and tension time integral in mechanically and chemically skinned segments of single muscle fibers, and the use of 31P and 13C nuclear magnetic resonance spectroscopy for dynamic studies of energy transformation and metabolism will be used in concert to test hypotheses quantitatively concerning the proposed specific objectives: (1) Establish conditions wherein a chemical energy balance is obtained in the various muscles studied; (2) To measure the energy cost of identifiable processes which are activated during contraction; and (3) To measure the efficiency of chemomechanical transduction in cardiac and skeletal muscle.